On Geological Characteristics And Metallogenesis Of Liaoshang Gold Deposit In Jiaodong Peninsula, Shandong Province

Located in Guanshui Town of Muping District in Yantai,Shandong Province,the Liaoshang gold deposit is the first discovered oversized gold deposit in eastern Jiaodong for the past few years.It takes up an impotant position in Jiaodong gold mineralization with complicated geotectonics due to the specific location on the conjunction part between North China plate and Sulu-Dabie orogenic zone namely the Jiaolai Basin.Most outcropping strata are the Paleoproterozoic Jingshan group and the Mesozoic Qingshan group with a small number of EW and NW trending formations,and the nearly EW and NE fault system is the major ore-controllng structure.The main outcrops namely the weak geissic fine-medium-grained garnet-bearing monzonitic granites of the Jiuqu unit of the Linglong sequence in early Yanshan period are the gold mineralization related orebodies and are distributed on the footwall of the Guocheng fault and the lateral of the northeastern margin of the Jiaolai Basin,or as stocks in the Jingshan strata.The Mesozoic vein rocks mainly contain amphibole syenites,orthophyres,dioritic porphyrites,and lamprophyres,extending on the NEand NNE trend.The strata composition of the studying area is relatively simple,and the predominant is the Jingshan group whose lower Xiangshan member is a set of Mg and Fe abundant clasitic sedimentary rocks with interbedded limestones-tremolite granulites,amphibole biotite granulites and amphibole granulites with marbles,and whose upper Dingguosi member is mainly marbles with minor tremolites and local amphibole granulite and amphibole biotite granulite laminas;marbles are one of the major home rocks.In the occupied two geological structures,the NE trending fault structures and interlayered detachmen structures are related to gold mineralization.The former faults,most found in the central orefield and represented by F2,develop granitic cataclasites which experience weak ferritization,local silicification and pyritization,and weak chloritization following the fissuresurfacies;the faults are characterized by compression,and control the orebody yield together with the lateral secondary faults and nearly parallel fissure zones.The latter structures,controlled by the orebody contact with the strata,mostly develop in the Paleoproterozoic Jingshan group,locally pass through the Linglong granites,and are composed of a group of basin-toward and nearly parallel detachment faults of breccias and catacliclasites with tectonic lens,where monzonitic granites and quartz veins can be found;the interlayered slip faults show the characteristics of normal faults and control several ore-controlling structures.The area mainly outcrops weak gnessic fine-medium-grained garnet-bearing monzonitic granites and a large quantity of vein rocks which obliquely cut the gold orebodies and are primarily dioritic porphyrites and lamprophyres,and secondly orthophyres,amphibole syenites,amphibole diorites,quartz veins in the NE trending,SE and NW inclination and with sharp dip angles.The pyrite dolomite veins themselves are gold orebodies.The Liaoshang deposit delimitates seven metallogenic alteration zones discovering 55 gold orebodies composed of five essential orebodies,five minor orebodies and 45 sporadic orebodies.For the orebodies,most takes on layers,lens,sphenoids,saddles,veins and so on with the general trend of 37°,SE inclination,the dip angle of 5°??55°,the orebody length of 80?380m,the inclination continuity of 42?171m,the orebody thickness of 1.36?42.93m and the average grade of 1.36×10-6?22.68×10-6.The essential orebodies are totally located in deep III alteration zone in which III-9,as the lagest single gold orebody,covers 50.7%of the total resource.The Liaoshang gold ores are all primary ones often buried below-50m including the major pyrite carbonate vein monzonitic granite ores,the pyrite carbonate vein bistagite marble ores and the secondly pyrite carbonate vein ores.The ores are characterized by their pyrite dolomite veins which differ them from the Jiaojia Type(shuttered zone alteration)and the Linglong Type(sulphide quartz veins),and this study tries to name the Liaoshang deposits pyrite carbonite vein deposits manifesting as thin veins and wide distribution.The rock veins are mostly irregular in shape at the width less than 1cm and are distributed in the tension fissures;many microveins less than 0.1mm can often be discovered under microscope to transect the protolith fissures,and the thickness near lm only can be seen in the workings.The ores always take on idiomorphic-hypidomorphic grained,granoblastic,cataclastic,porphyoclastic,broken-granied and metasomatic textures,and veinlet,veinlike,massive,disseminated,lumpy and brecciated structures.Pyrite is dominant in metallic minerals while some natural gold,galenas,chalcopyrites,magnetites and pyrrhotites can be identified in the polished sections;the nonmetallic minerals are different according to ore types,mainly including quartz,potash feldspars,plagioclases,dolomites,calcites,bistagites and sericites in which the dolomites resulting from later filling function are dominant.Most ores in deep and peripheric Liaoshang are primary ones.By field invesitigation,hand-sample observation and rock-mineral determination,the hydrothermal mineralization can be divided into three periods:the gold-dolomite-pyrite period,the gold-dolomite-polymetal sulphide period and the calcite-quartz period,in which period I and II are more important.The roof and floor surrounding rocks of the Liaoshang gold orebodies are mainly monzonitic granites,bistagite marbles,and amphibole granulites.The boundaries are not clear with local weak mineralization zones.The surrounding rock alteration is highly developed mainly consisting of silicification,sericitization,choloritization and pyritization in which most happen earlier than the main mineralization period except for the veined fulling-flunction-led carbonatization and pyritization.The fluid inclusion study shows that the metallogenic temperature of the Liaoshang gold minerlization happenes in the medium toward low temperature environment of 199.8-247.1?.The researches of S-Mg-C-O isotopes and lamprophyre features indicate that the Au metallogenic materials are mainly mantle-derived,Mg,C and 0 in the pyrite dolomites may come from the dolomite marbles of the Jingshan group,and the associated vein rock magma may supply S to mineralization;metallogenic fluids mostly come from atmospheric precipitation and a little from mantle.In conclusion,the Liaoshang gold deposit is defined as the middle toward low hydrothermal veined deposit controlled by strata and mantle-derived magmation.The gold-enriched fluids from the deep femic magmation and the underground-water deep circulation heated by the magmation continuously replace marbles and fill the secondary faults and the fissures which gradually develop into pyrite carbonate veined gold deposits namely the Liaoshang type deposit.